746 research outputs found
Draco -- A Failure of the Tidal Model
We test whether the structural properties of the nearby dwarf spheroidal
(dSph) galaxy Draco, a well-studied Milky Way companion, can be reconciled with
the suggestion that dSphs are unbound tidal remnants with a large depth extent
along the line of sight. In order to apply the observational test of this
hypothesis suggested by Klessen & Zhao, we use public photometric data from the
Sloan Digital Sky Survey (SDSS) to explore the width of Draco's blue horizontal
branch over a range of areas covering 0.06 square degrees to 6.25 square
degrees centered on Draco. The SDSS database is the only currently existing
database with sufficient depth and area coverage to permit a stringent test of
the tidal models. We show that blue horizontal branch stars extend beyond the
previously inferred limiting radii of Draco, consistent with the observed
absence of a truncated stellar surface density profile of this dSph. We
calculate new models for a galaxy without dark matter, using Draco's
morphological properties as constraints. The resulting models are unable to
reproduce the narrow observed horizontal branch width of Draco, which stays
roughly constant regardless of the field of view. We conclude that Draco cannot
be the remnant of a tidally disrupted satellite, but is probably strongly
dark-matter dominated. (ABSTRACT ABBREVIATED)Comment: 26 pages, 9 figures included, accepted for publication in ApJ,
high-resolution version available at
http://www.aip.de./~ralf/Publications/p22.abstract.htm
CN abundance variations on the main sequence of 47 Tuc
We report on a deep spectroscopic survey for star-to-star CN variations along
the main sequence (MS) of the globular cluster 47 Tuc with ESO's VLT. We find a
significant bimodal distribution in the S(3839) index for main-sequence stars
in the mass range of ~0.85 to 0.65 M_sun, or from the main-sequence turn-off
down to ~2.5 mag below the main sequence turn-off. An anti-correlation of CN
and CH is evident on the MS. The result is discussed in the context of the
ability of faint MS stars to alter their surface composition through internal
evolutionary effects. We argue against internal stellar evolution as the only
origin for the abundance spread in 47 Tuc; an external origin such as pollution
seems to be more likely.Comment: 29 pages, including 8 figures; Accepted for publication in AJ,
scheduled for Januar
Evolutionary instability of selfish learning in repeated games
Across many domains of interaction, both natural and artificial, individuals use past experience to shape future behaviors. The results of such learning processes depend on what individuals wish to maximize. A natural objective is one’s own success. However, when two such “selfish” learners interact with each other, the outcome can be detrimental to both, especially when there are conflicts of interest. Here, we explore how a learner can align incentives with a selfish opponent. Moreover, we consider the dynamics that arise when learning rules themselves are subject to evolutionary pressure. By combining extensive simulations and analytical techniques, we demonstrate that selfish learning is unstable in most classical two-player repeated games. If evolution operates on the level of long-run payoffs, selection instead favors learning rules that incorporate social (other-regarding) preferences. To further corroborate these results, we analyze data from a repeated prisoner’s dilemma experiment. We find that selfish learning is insufficient to explain human behavior when there is a trade-off between payoff maximization and fairness
Chemical Evolution in the Carina Dwarf Spheroidal
We present metallicities for 487 red giants in the Carina dwarf spheroidal
(dSph) galaxy that were obtained from FLAMES low-resolution Ca triplet (CaT)
spectroscopy. We find a mean [Fe/H] of -1.91 dex with an intrinsic dispersion
of 0.25 dex, whereas the full spread in metallicities is at least one dex. The
analysis of the radial distribution of metallicities reveals that an excess of
metal poor stars resides in a region of larger axis distances. These results
can constrain evolutionary models and are discussed in the context of chemical
evolution in the Carina dSph.Comment: 3 pages, 2 figures, to be published in the proceedings of the
ESO/Arcetri-workshop on "Chemical Abundances and Mixing in Stars", 13.-17.
Sep. 2004, Castiglione della Pescaia, Italy, L. Pasquini, S. Randich (eds.
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